This time of year, especially when it gets cold outside in the evenings, we get many calls from homeowners concerned that there is cold air blowing out of the duct registers in their home when their heat is running. We wanted to address this, as many times homeowners think this is an issue with their heating and cooling system, but if they have a heat pump, it's actually typically a very normal process. Don't know if you have a heat pump? Check out this article, on how to tell if your unit is a heat pump or air conditioner. In the summer, a heat pump picks up the heat in your home and dumps it outside. In the winter, your heat pump picks up heat from the outside and dumps it into your home (yes, even when it's cold outside). Now, that may be hard to grasp, but this is the way it works. In the summer, your indoor coil is cold (and pulls humidity out of the air, and comes in the form of water) which is drained from that PVC pipe located on your roof, or side of your house.

In the winter, that same humidity is pulled out of the air, but it comes in a form of frost on the outdoor coil (condenser). If you see frost on the outdoor coil, it is natural to the process and it is not a sign of any kind of malfunction- the reason the coil is cold falls back to the second law of thermodynamics- which states that energy always moves from areas of higher concentration to areas of lower concentration. As the frost builds up on the outdoor coil, it could potentially inhibit the ability for air to flow across that coil. Once the frost builds up, your heat pump automatically goes through what we call, a defrost cycle to keep allowing the air to flow across the coil freely (which is a necessary function of your system). You may have noticed a loud sound right before cold air blows out through the vents in your home. The sound you hear is the reversing valve switching your unit over to cooling mode. Frost on the outside coil is condensation that forms up on the coil in the form of frost.

When a heat pump begins the defrost cycle, the outdoor fan shuts off and the system switches over to cooling mode. The reason for this, is that in the cooling mode, the outdoor coil actually becomes hot. This is because hot refrigerant circulates through the outdoor coil, which in turn melts the frost. The reason the fan shuts off during this process is so the system doesn't pull the cold air from the outside across the coil while the hot refrigerant circulates. While the system is in cooling mode, the frost that had built up on your outdoor coil then turns to water, runs down the outdoor coil, and piles towards the bottom. When the defrost cycle is complete and the defrost sensor closes, the system runs in heating mode again. This is when the fan kicks back on and all of the frost that ran down to the bottom of the coil in the form of water is pushed out of the top of the unit, sometimes in the form of steam. If you see steam coming out of your outdoor coil that is a normal operating procedure for a heat pump system, so don't be alarmed.

Don't panic if you have a heat pump and if cold air blows out of your duct work for a little while on and off while you run your heat at home. Don't panic if you see frost on your outside equipment's coil. split ac unit componentsDon't panic if steam comes out of the top of the outside equipment. fitting air conditioning unitsIt's all part of the process of heating your home with a heat pump!ac window unit fanPosted on Aug 6 2014 by It's easy to see that this duct system is zoned because parts of the zone dampers are visible on the outside of the ducts. The green lights indicate that the zone dampers are open. Last week I wrote about what happens when you try to save energy by closing air conditioning registers in unused rooms. In the end, I recommended not doing it because you won’t save money and you may create some big problems for yourself, like freezing up the coil and killing your compressor.

At the end of the article, I mentioned that zoned duct systems do close off registers, and that doing so can be OK with the right kind of equipment and design. But there’s one thing often done in zoned duct systems that’s rarely done well. Before we find out what that thing is, though, let's be precise in our language and clear up exactly what we're talking about. The word “zoning” is used in more than one way in the context of heating and air conditioning systems in a house. First, larger houses are almost always zoned. That is, they have more than one thermostat so you can control the conditions separately in different parts of the house. In a two-story house, for example, there will probably be at least two thermostats — one upstairs and one downstairs. Keeping Cool in a Two-Story House How Duct Leakage Steals Twice Thou Shalt Commission Thy Ducts! The other way that the term “zoning” is used is to describe a single duct system attached to a single HVAC(Heating, ventilation, and air conditioning).

system that serves multiple zones. In most homes with forced-air HVAC systems, each thermostat is connected to its own heating and cooling system. The home is zoned, but the HVAC system is not. In a “zoned system,” a single heating and air conditioning system is controlled by multiple thermostats in multiple zones. In the photo above, the three green lights are part of three zone dampers that control the flow of air to three separate zones. Depending on the needs of the house, any combination of one, two, or three zone dampers may be open and sending conditioned air to their respective zones. If only one or two of the zones are calling for air, most air handlers will create extra static pressure because one or two of the pathways are closed off. Enter the bypass duct (shown in Image #2, below). When the system is running but not all of the zone dampers are open, the bypass duct — in theory — is supposed to relieve the extra pressure and maintain good air flow throughout the duct system.

The problems with the bypass duct A few years ago at the ACI conference, I heard John Proctor and Rick Chitwood discuss the issue of bypass ducts. Proctor isn’t a fan of zoning at all, and Chitwood is. On one point, though, they both agreed: Bypass ducts should never be used. Here are three reasons why: Throwing cold air directly into the return plenum reduces the temperature of the air coming in to be cooled. That makes the evaporator coil get colder, and the colder it gets, the less efficient it becomes. The bypass duct steals air. Even with all three zone dampers open, the bypass duct has a big pressure difference across it, and air is lazy. It'll cheat and take the path of least resistance whenever possible, in this case the bypass duct. Not only is a colder evaporator coil less efficient, it's also more likely to freeze up, as the condensation it collects eventually drops below the freezing point. (And if you think a bypass duct is bad for air flow, a frozen coil is way worse.

It's really hard to push air through a solid block of ice.) Savings from eliminating the bypass Just this week, Proctor posted an article on zoning and bypass ducts on his website. With the article, he included a video demonstration of a zoned system, showing the changes in airflow and temperatures with and without the bypass duct open. Then he performed the calculations to show the efficiency for each configuration. In his little experiment, the three configurations with the bypass duct closed (with no air through bypass) were 22%, 27%, and 32% more efficient than the configuration with the bypass duct open. Of course, if you’re sending air to only one zone, you still have the issues of reduced air flow in a PSC blower and increased energy with an ECM blower, as I described last week for the register-closing scenario. To do zoning right, you’ve got to account for the extra air when one or more zones are closed during operation. Probably the best way to do that is with a multi-stage air conditioner that can also ramp down the fan speed to send less total air through the system.